Tumor necrosis factor (TNF) is a cytokine with the potential for the treatment of cancer that also promotes immunity, metabolic changes that accompany disease states, insulin resistance, inflammation, angiogenesis and wound healing. To realize the potential of TNF as a therapeutic agent, to attenuate its pathological activities and promote its beneficial effects requires insight into how TNF works. The first step in TNF action is binding to either of two receptors. The type 1 TNF receptor (TNFR1) promotes apoptosis in malignancies, fibroblast proliferation, antiviral responses, activation of a group of transcription factors, and plays a predominant role in the host defense against microorganisms. For this reason, this proposal focuses on how TNFR1 mediates its effects. Most recent work has focused on association of TNFR1 with a group of intracellular proteins that contain a death domain motif and the role of these proteins in TNF action. My laboratory has found that the TNFR1 signaling complex is composed of more proteins than previously appreciated. We have found that the TNFR1 complex contains Jak kinase, c-Src, a non-receptor tyrosine kinase, SHP-1, a protein tyrosine phosphatase, phosphatidylinositol 3-kinase, a lipid kinase, and Rac1, a GTPase. These newly identified components of the TNFR1 complex play a role in activation of NF-kappaB, a transcription factor important to immunity and cell survival. It is likely that Rac1 plays a role in coupling TNFR1 to activate p38 MAPK and JNK kinases that activate transcription factors important to stress responses. The aims of this proposal are to determine how the TNFR1 complex forms and couples to pathways that generate TNF responses and how the complex determines whether cells live or die.